I could apply your math to a software crop of a FF image (same lens, same camera) and come to the same conclusion which would be incorrect because the blur must be the same throughout the entire FF image.

Correct. If you cropped in software, the reduced angle of view would still mean a relatively greater impact of the existing blur, which might take it from below threshold to above threshold for detectability. In the same way, cropping an image in post affects sharpness and DoF.

I could apply your math to a software crop of a FF image (same lens, same camera) and come to the same conclusion which would be incorrect because the blur must be the same throughout the entire FF image.

Correct. If you cropped in software, the reduced angle of view would still mean a relatively greater impact of the existing blur, which might take it from below threshold to above threshold for detectability. In the same way, cropping an image in post affects sharpness and DoF.

I may just be quibbling, or I perhaps I'm just wrong, but I'm not comfortable with your phrasing. It's not the cropping per se that causes these effects. Rather, it is the greater enlargement of the smaller image (usually to achieve the same viewing size) and that is what the CoC is a measure of.

The 1 / focal length guide applies to 35mm equivalent focal length, so for crop sensor it does get that bit worse.

Why would that be true? Not questioning your personal experience but what's the technical reason for that. Possibly because with a crop-body you're standing further away for the same framing, so the angle of view is less, and vibrations due to hand shake would then cross a greater angle. Ok, true assuming same framing which is the case 99% of the time.

Because the 1/focal length approximation derives from the relationship between the angular motion and the angle of view. Thus, at wide angles, where a given amount of angular motion represents only a small fraction of the angle of view, the effects of camera shake are less likely to have an impact on the image. Since the crop factor reduces the angle of view, you need to account for that in the 1/focal length approximation.

If you want to even up blur as percent of total image then apply the crop factor.

If you want to get what you can out of the sensor then you need to use factors relating only to pixel density and crop could end up needing less shutter speed than FF in rare cases (10D vs 5D2).

Let me put it this way - Canon's viewpoint is that the crop factor impacts the 1/focal length rule. With a 200mm lens on a 5DII in Av mode and Auto ISO, the camera will maintain 1/200 s (reciprocal of focal length) and increase the ISO, and only after hitting ISO 3200 will the shutter speed begin to drop lower than 1/200 s. In the same situation on a 7D, the camera will maintain 1/320 s (reciprocal of 1.6 x focal length) until ISO pegs at 3200.

One thing is different however; circle of confusion is smaller on an crop sensor because we enlarge more for any given print size. Is that the actual physical phenomenon at play... when the angular motion that occurs during the exposure is larger than the CoC we begin to see the blur?

With the current 100-400L model, what is the minimum shutter speed you need at 400mm with IS to get a decent keep rate for your picture? I assume it would be less then 1/400. Maybe 1/200 with the current IS? Any thoughts as to how much of an improvement a new IS version would make using these parameters?

I was always afraid that at 400mm and f5.6, even with IS it must not be always obvious to get great shot on a cloundy day?

I've used it in some adverse lighting conditions and been able to use shutter speeds as slow as 1/30 at 400mm and get usable results, but not the *best* keep rate. Settling for 1-stop darker photos for lightening in post or balancing that change with ISO, I got consistently great results with 1/60.

When I bought it, I expected to only be able to use it in blazing sunlight. But I've used it in all kinds of circumstances now, and have been really happy with the results.

Edit:Here is an example. This was taken on my backup camera, a Rebel XTi. 400mm, F5.6, 1/50. This is a shot of the woman as reflected in a mirror. I can't take credit for the image myself, it was taken by one of my contract photographers during the course of our shoot that day. Very reasonable quality coming from an older entry-level camera, a long lens, and less-than-ideal lighting.